The present invention relates to friction drive mechanisms particularly suitable for use in power driven vehicles such as riding lawn mowers. It is typical for such drive mechanisms to utilize a driving disc operated by the output of an engine to drive a driven wheel or roller which is connected to the axle of the ground-engaging wheels by means of a transmission so as to power-drive the vehicle over the ground surface. The driven wheel has fixed on its periphery, a friction ring typically made from rubber material for engaging the surface of the driving disc with sufficient friction to propel the vehicle.
Several factors combine to produce high acceleration during the initial forward motion of the riding mower. Among these factors are: the engine power is dictated by the grasscutting function of the machine and this power is much greater than needed for propulsion alone; the transmission ratio from the engine to the wheels is relatively high because the ground speeds of the machine must be low, and this speed reduction is accompanied by a corresponding multiplication of torque available to the wheels; the machine is equipped with a hand throttle that is fixed to a preselected speed by the operator and is not variable during start-up as with an automobile; the throttle is generally set at full or near full engine speed to best accomplish the grass cutting function; and, the relative high friction between the driving disc and the driven wheel necessary for proper function and durability.
The result of high starting acceleration is jerky operation which can cause operator discomfort and annoyance, as well as vehicle instability in circumstances of improper use by the operator. Moreover, this jerky operation contributes to accelerated wear and early failure of transmission components.
Attempts have been made in the past to overcome the aforementioned conditions. For example, U.S. Pat. No. 3,667,304 to Puffer et al discloses a friction drive for a riding lawn mower which utilizes two springs exerting different biasing forces for urging the driven wheel against the driving disc. One spring initially engages the driven wheel with the disc with a first spring force and the other spring is operable to increase the biasing force between the roller and the disc after initial engagement of the roller with the disc under the influence of the first spring. Preferably, the second spring has a spring rate greater than the spring rate of the first spring. This spring system is rather complicated and it is difficult to control the biasing force against the driven wheel.
Another attempt to provide a smooth starting drive mechanism for a lawn mower is disclosed in U.S. Pat. No. 3,706,363 to Niles. It discloses a plurality of clutches between the driving disc and the driven wheel for initially providing slippage in the clutches. However, this system decreases the useful service life because of the increase in the number of parts and the number of wearing surfaces on the clutch members and the driven wheel. Moreover, the clutch discs require, for their engagement, axial forces parallel to their mounting shaft which tend to cause binding after sufficient usage. Also, the system of Niles patent may not be applicable to existing lawn mower mechanisms without substantial modification.